Position tracking servo control systems and methods

ABSTRACT

The invention is a transducer carriage servo control system and subsystem for a data disk storage system providing markedly improved carriage response time and positioning accuracy. The servo control system uses for feedback control a position error signal having an amplitude dependent at all times upon the amplitudes of both of a pair of normal and quadrature signals indicating actual carriage position. Moving and stationary control is provided through a reference position generator held at a stationary value for stationary (track following) control and incrementally changed for movement control during an access mode of operation. A velocity profile function generator is used to control the incrementation of the reference position in the generator at a nominal modeled velocity rate. Feedback control of incrementation is also provided by the position error signal converted to a velocity error signal through a voltage controlled oscillator. The incrementing system includes a motor adaptive circuit measuring distance moved during an acceleration portion of an access operation and comparing the distance displaced with a predetermined value to further vary the frequency of the velocity error signal, thereby adjusting the modeled servo control system to actual system performance. The motor adaptive circuit is employable with other servo systems utilizing a signal having a variable characteristic for feedback control.

FIELD OF THE INVENTION

The invention relates to systems and methods for positioning a movablemember with respect to a path of movement used with the systems and inparticular, to servo control systems for positioning transducers in adata storage device for the transfer of data to or from a data recordstorage medium of the device.

BACKGROUND OF THE INVENTION

A typical positioning application to which the present invention relatesis the positioning of a data transducer or "head" over a selected trackof a magnetic disk file in a magnetic disk data storage system. Otheruses will be evident to those skilled in the art.

U.S. Pat. No. 4,068,269 to Commander et al. discloses a transducerpositioning system for a magnetic disk data storage system incorporatingplurality of disks and associated magnetic transducers or "heads" forreading and writing data on each disk. The transducers are ganged forsimultaneous movement by a single actuator. A single "servo" disksurface and associated "servo" transducer are dedicated to transducerpositioning control. The dedicated servo disk surface is prerecordedwith a plurality of concentric magnetic servo tracks of substantiallyuniform width which are arranged in alternate radial sectors andstaggered radially in an alternating fashion from one another. Eachmagnetic servo track is provided with at least one change in directionof magnetization. As the servo disk surface is rotated, the servotransducer generates a signal indicative of the magnetic transitionsoccurring in the servo tracks opposite it. The transducer generatedsignal is passed through appropriate circuitry which generates a firstor "normal" position signal and a second, "quadrature" position signal.The position signals are oscillatory about a zero voltage and 90 degreesout of phase with one another. Each of the two position signals isassociated with one of the two alternating sets of servo tracks. Each ofthe position signals is linear for approximately one quarter track widthto either side of the boundary of adjoining tracks in the set of sectorswith which the signal is associated. The two position signals arealternately linear as the servo head is moved radially across the servodisk surface. The normal position signal, which is selected to be linearabout each on-data track position, is used for transducer positioncontrol during track following operations when data is being read ontoor from the disks.

A positioning system must also control transducer movement between datatracks (and corresponding servo tracks) in an "access" or "seek"operation. The time taken to move the heads between selected tracks insuch a mode is generally known as the "access" time and is one of themore important performance characteristics of the positioning system. Tominimize the access time for a given mechanical configuation andactuator performance requires a positioning system which will controlhead movement velocity at an optimal level and bring the transduceraccurately to rest on the desired track.

In the aforesaid U.S. Pat. No. 4,068,269, access motion by thetransducer is accomplished by means of a continuous distance to gosignal generated by counting down the number of tracks between theoriginal position of the servo transducer and desired position of theservo transducer using track crossing pulses generated by logicidentifying the alternating linear portions of the normal and quadratureposition signals. The derived distance to go signal is passed to areference velocity signal generator which outputs a time-optimalreference velocity signal which is compared with the actual headvelocity signal derived by differentiating and piecewise combining thesuccessively linear portions of the normal and quadrature positionsignals.

In U.S. Pat. No. 4,115,823, also to commander et al., there is describedyet another positioning system for use with a disk data storageapparatus similar to that just described wherein the normal andquadrature position signals generated by a dedicated servo transducerand disk surface are combined with servo position signals generated by adata transducer from servo signals mixed with data signals on the datadisk surface. Again, the linear portions of the two position signals arealternatively differentiated and combined to generate a velocity signalused in head control.

There are several limitations associated with the positioning systemsdescribed in the U.S. Pat. Nos. 4,068,269 and 4,115,823. First, onlyone-half of the available servo position information is utilized as onlythe linear portions of the normal and quadrature position signals areused in positioning the transducer. Next, servo track widths areidentical to data track widths. As data track widths are made narrowerby various techniques to increase data density, the servo tracks must besimilarly narrowed. As the servo tracks are contiguous and extendentirely across the servo disk surface, this becomes more expensive toaccomplish. Moreover, as data and servo tracks are recorded withnarrower widths, the described positioning systems become moresusceptible to mechanical perturbations such as eccentricity which maydrive the servo transducer into the non-linear region of the normalposition signal or trip the transducer onto an adjoining servo track.

Other disadvantages arise in the described systems. Accurate positioningbecomes difficult during transducer movement because noise in the systembecomes predominant when the positional signal is differentiated at lowvelocity, as when the transducers are approaching their final position.In differentiation type systems such as have been described, variationsin the level of the position signal can similarly cause difficulties.Such variations may be caused by fluctuations in transducer fly heightwith respect to the disk. As a result, the smoothness of the disk'ssurface must be held to very tight tolerances. Each of the describedCommander et al. systems is also sensitive to position signal wave formlinearity. Any deterioration of the servo head or associated electronicscan effect the linearity of the positional signal wave forms and causeserious control problems. This requires the imposition of stringentmanufacturing tolerances with respect to the components associated withthe servo control system.

Bandwidth requirements imposed on positioning control systems duringaccess-type operations may be significantly reduced by the use offeedforward control. U.S. Pat. No. 4,200,827 to Oswald describes afeedforward/feedback transducer positioning system used in a magneticdisk data storage device. In feedforward control, a primary current isapplied to the actuator moving the heads. The primary current is onewhich would accomplish an optimal or near-optimal movement of the headsin an ideal or nominal model of the electromechanical servo mechanismbeing used. Variations between the actual performance of the system andthe modeled or ideal performance upon which the primary current is basedis compensated for by introducing small perturbations into the primarycurrent as feedback control.

U.S. Pat. No. 4,200,827 describes a "bang-bang" access servo controlsystem in which the heads are moved by the control system at near themaximum acceleration and deceleration which the electrical andmechanical components of the system can tolerate. For long accessmovements, the heads "coast" at maximum velocity between accelerationand deceleration modes. The control system generates a drive current (orfeedforward current) which can be controllably switched in sign formovement of the heads in either direction along a radius of the disks.Before being fed to the actuator motor moving the heads, the drivecurrent is combined with a feedback control current proportional toerror occuring in the access operation. Two embodiments are described,one utilizing velocity error and the other utilizing position error togenerate the feedback control current. In the former embodiment atransducer head velocity signal is generated by differentiating a singlecyclically varying servo position signal (i.e. normal position signal)generated by a dedicated servo transducer and associated servo disksurface. During the non-linear portions of the servo position signal,actuator current, which is proportional to acceleration, is integratedand used as a measurement of velocity. In the latter embodiment, one ortwo periodic servo head position signals (i.e. normal or normal andquadrature signals) are generated by the dedicated servo head andassociated servo disk surface. A reference position signal is generatedby integrating a reference velocity signal. The reference positionsignal is then phase compared with the servo head position signal togenerate a position error signal and a proportional position errorcurrent which is combined with a drive current, as in the velocitycontrolled system, to provide a varying current to the actuator. In apreferred embodiment of the position error control system, both normaland quadrature servo head position signals are generated and the linearportions of each phase compared in a sequential, alternating fashion (aswas done in the Commander '269 and '823 patents) with correspondingnormal and quadrature reference position signals. The latter aregenerated by integrating and then phase shifting a single velocityreference signal.

As the invention of the U.S. Pat. No. 4,200,827 patent determines servoposition by combination of normal and quadrature signals in the mannerof the two aforementioned Commander et al. patents, it suffers the samedrawbacks. Additionally, where velocity error is used as the feedbackcontrol mode, actuator motor performance must be tightly controlled topredicted nominal conditions or errors are introduced to the measuredhead velocity signal generated by integration of the motor current. Thiserror is cumulative during each access operation and makes landing ontrack at the end of the operation problematical at best. As a result,very tight manufacturing and reliability tolerances are imposed on theactuator motors which must be used with this system.

U.S. Pat. No. 4,297,734 to Laishley et al. describes yet another servopositioning system for data disk systems utilizing feedforward plusfeedback control with sampled, rather than continuously generated normaland quadrature servo position signals. This control system is subject tothe same problems which beset the previously identified systems,particularly the requirement that actuator motor performance be tightlycontrolled during manufacture and monitored over the life of the system.As servo position is only periodically sampled and not continuouslymonitored, small variations of the actual actuator motor performancefrom modeled motor performance can introduce significant errorsdegrading feedback control.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a servo control method andapparatus for controlling the movement and positioning of a movingmember which has decreased sensitivity to variations in the performancecharacteristics of the servo system components.

It is yet another object of the invention to provide a servo positioningcontrol method and apparatus using only measurement of servo positionfor moving member control during movement whereby the differentiation ofservo position or the integration of actuator current to generate aservo velocity representation is avoided.

It is yet another object to provide a servo control method and apparatusfor controlling the positioning and movement of a movable member using aposition error control signal, the magnitude of which is dependent atall times upon the magnitudes of both of a pair of cyclical and phaserelated servo position signals indicating actual position of the movablemember.

It is yet another object of the invention to provide such a servocontrol method and apparatus further including feedforward and feedbackcontrol during the movement of the movable member.

It is yet another object of the invention to provide a new servo methodand apparatus for feedforward control of a moving member which isself-adapting to the actual performance of the servo system.

It is yet another object of the invention to provide a method andapparatus for adapting a servo control system to the actual performanceof the servo system.

SUMMARY OF THE INVENTION

The invention is described in terms of a disk data storage systemincorporating a new servo control system utilizing position errorfeedback control for extremely rapid and accurate positioning of datatransducers with respect to the data disk. The described, preferredembodiment of the invention is implemented by means of discrete circuitelements for parallel processing and the fastest possible response time,but it is recognized that one or more microprocessors may be substitutedfor the individual elements to achieve similar accuracy but with alonger response time. The described disk data storage system includes aplurality of data disks mounted for simultaneous rotation, a pluralityof transducers for at least reading data recorded on the disks, acarriage mounting the transducers for simultaneous radial movement withrespect to the disks, a voice coil motor coupled to the carriage formoving the carriage and a control system generating a motor drivecurrent supplied to the motor for controlling the positioning of thecarriage. A disk surface and an opposing transducer are allocated forservo control. Recorded on the surface are servo signals which define aplurality of contiguous servo segments or "bands" on the surface in theradial direction and indicate incremental position within a band whenthe system is operated. The signals are recorded so as to be detectableby the dedicated transducer from any other information which may berecorded on the dedicated disk surface. It is envisioned that thecontrol system may be implemented in any of a wide variety ofapplications which, broadly stated, include a member movable along adefined path of movement, means for dividing the path of movement into aseries of contiguous segments, means for indicating actual incrementalposition of the member with respect to segments, actuator means coupledto the member for movement and a control system of the type to bedescribed, controlling the operation of the actuator.

A most important aspect of the invention is the control system whichemploys position error feedback control. The components of thedescribed, preferred embodiment control system include a digitalresolver controller having an incrementable counter for storing adigital reference position indicating an approximate position of theservo transducer with respect to the servo bands. The digital referenceposition includes an integer portion indicating an integer number ofbands, and a fractional portion indicating an incremental positionwithin a band. The control system also includes a position detectorcircuit coupled to the servo transducer for generating from the detectedservo signals a pair of so-called "normal" and "quadrature" positionsignals. The two position signals are both trigonometric, having thesame oscillatory form, and a fixed phase displacement with respect toone another, and together indicate unambiguously the actual incrementalposition of the servo transducer with respect to a servo band. Theremaining portion of the digital resolver controller is dedicated tocircuitry generating a position error signal indicating a phasedifference between the actual incremental position indicated by the twotransducer position signals and the approximate incremental positionindicated by the fractional portion of the digital reference position.Unlike other position control systems, the magnitude of the positionerror signal is dependent at all times upon the magnitudes of both ofthe two transducer position signals. In particular, the fractionalportion of the digital reference position value is converted into a pairof trigonometric function values which are multiplied with the positionsignals and combined to generate a trigonometric position error signalhaving an oscillatory, phase dependant form, the phase being thedifference between the two incremental positions in phase format. Theservo control system also includes circuit elements responsive to theposition error signal for generating a drive current supplied to themotor for positioning the carriage.

The system described thus far can be used to hold the carriage at afixed position by equating the digital reference position to the fixedposition. For movement of the carriage an updating subsystem is providedfor controlled variation of the stored reference position. Inparticular, the updating subsystem increments the counter storing thereference position value from an initial position value to the finalposition value. The updating subsystem in the described embodimentincludes a summing subcircuit formed by a plurality of stacked adderswhich sum the stored reference position value with a predeterminedcomplementary value related to the desired final position such that whenthe digital reference position is equal to the final reference position,the sum equals a predetermined value. The output of the adders in thesumming subcircuit is indicative of the distance between the storedreference position and the commanded or final position. The updatingsubsystem further includes programmable read-only memories responsive tothe distance indication outputted by the summer circuit. The memoriesare programmed to output a scaling function, also referred to as thevelocity profile function, which has a magnitude dependent upon themagnitude of the summer circuit output and thus distance between theindicated reference position and final position. In particular, thevelocity profile signal also has a magnitude proportional to themagnitude of a desired nominal velocity of the carriage when undergoinga time optional movement and located at a distance from a desiredterminal position equivalent to the distance between the stored digitalreference position and the commanded final position. The updatingsubsystem also includes a rate multiplier outputting a cyclic signalhaving a cycling frequency related to the magnitude of the velocityprofile function. The cyclic signal is used to increment the countersstoring the digital reference position.

Because of a tendency for the described system to increment the digitalreference position during the acceleration portion of a carriagemovement faster than the actuator can respond to the position errorsignal, the updating subsystem includes an override circuit whichreverses the direction of incrementation of the stored referenceposition away from the commanded, final position when the absolute valueof the position error magnitude approaches a predetermined value. In thedescribed embodiment, this is accomplished by providing a pair ofvoltage comparitors, having as their input the position error signal.Output from the voltage comparitor is used to generate a signal to thecounter storing the digital reference position which controls thedirection of incrementation of the counters. Other implementations ofthe update circuit are known to be possible.

To reduce system bandwidth requirements, the positioning system includesfeedforward control. A feedforward seek drive circuit is provided togenerate a feedforward drive signal for gross control of the positioningsystem during a seek movement. The seek drive circuit provided includesa programmable read-only memory responsive to the velocity profilesignal previously referred to. The PROM stores a table of decelerationvalues also referred to as feedforward scaling functions, whichcorrespond to the magnitude of an electric current needed for driving amodel positioning system utilizing a nominal, model actuator in a timeoptional motion. A reference voltage signal is generated which switchesin sign between acceleration and deceleration in response to componentsof the position error signal. The reference voltage signal andfeedforward scaling function are combined to generate a drive signal.The described embodiment further provides a gain control circuit forreducing the gain of the position error signal during a seek operationand a summing junction for combining the gain control position errorsignal with the feedforward drive signal to create a composite drivesignal actually used to generate a drive current for the motor. Whilegain of the position error signal is reduced during a seek movementoperation, it is not entirely removed as it is believed that thepresence of the signal, even at a reduced level, provides a smoothingeffect as the carriage approaches the final position.

The described embodiment additionally uses the position error signal asfeedback control for the reference position incrementing step. This isaccomplished by providing a voltage control oscillator outputting acyclic signal having a nominal cycling frequency and varying about thenominal frequency in relation to the magnitude of the position errorsignal. A sign change circuit passes or inverts, depending upon thedirection of motion of the carriage, the position error signal forwardedto the oscillator to relate the passed signal to a velocity error asopposed to a position error. The implementation of position error signalfeedback control in the reference position incrementing process reducesthe likelihood of a cycle limiting condition occurring.

Another important aspect of the invention is a motor adaptive circuitprovided between the voltage control oscillator and the aforesaidincrementing rate multiplier to further vary the frequency of the cyclicsignal outputted by the oscillator in relation to the actual performanceof the positioning system. In this way, systematic long-term errorsarising from manufacturing tolerance variations, wear or otherdeterioration of the positioning system components can be separatelycorrected for, thereby providing a full range of positioning controltailored to the actual components of the positioning system. Thedescribed embodiment operates by measuring displacement of the referenceposition over a predetermined time period during an acceleration portionof the carriage movement, and computing a ratio of the measureddisplacement of the system against the maximum possible displacementachievable by the fastest possible positioning system. The ratio is fedinto a second rate multiplier and is used to scale the frequency of thevoltage control oscillator signal before it is passed to theincrementing rate multipier also receiving the velocity profile functionas an input.

The motor adaptive circuit constitutes a second and separable aspect ofthe invention. It is envisioned the circuit can be used with otherdynamic servo positioning systems which utilize an error signal forfeedback control having a signal variable indicative of the erroroccurring during the movement.

These and other important aspects of the invention will be understoodupon examination of the accompanying figures and detailed description ofthe preferred embodiment of the invention.

BRIEF DECRIPTION OF THE FIGURES

A preferred embodiment of the invention will now be described withreference to the accompanying drawings in which:

FIG. 1 shows schematically a disk data storage apparatus incorporatingthe invention;

FIG. 2 shows the wave forms generated from recorded servo signalsdetected by a servo transducer and an associated position detectingcircuit; the wave forms are a pair of cyclical, sinusoidal transducerposition signals having a fixed phase relationship with respect to oneanother and indicate the incremental position of the servo transducerwith respect to a servo band;

FIG. 3 depicts schematically the major components of the preferredcontrol system of the present invention;

FIGS. 4a and 4b depict schematically the components of a digitalresolver controller ("DRC") of the control system;

FIG. 5 depicts schematically a dual gain amplifier circuit of thecontrol system;

FIG. 6 depicts schematically a summer circuit of the control system;

FIG. 7 depicts schematically a scheduler circuit of the control system;

FIG. 8 depicts schematically a feedforward seek drive ("FFSD") circuitof the control system;

FIG. 9 depicts schematically the sign change ("±1") circuit of thecontrol system;

FIG. 10 depicts schematically a voltage controlled oscillator circuit ofthe control system;

FIG. 11 depicts schematically a motor adaptive circuit of the controlsystem;

FIG. 12 depicts schematically a rate multiplier circuit of the controlsystem; and

FIG. 13 depicts schematically an override circuit of the control system.

DETAILED DESCRIPTION OF THE INVENTION Overall System

A data disk storage apparatus incorporating various embodiments of theinvention hereinafter described is shown schematically in FIG. 1. Aplurality of magnetic disks 20 are mounted in a stacked orientation forrotation about a central axis 22. A data transducer or head 24 isassociated with the upper or lower surface of each disk 20 for recordingdata on or reading data from the disk. One disk surface 21 is allocatedto servo control and has servo positioning information recorded thereon.A servo transducer or head 26 is associated with the servo surface 21.The transducers 24 and 26 are ganged on a carriage 27 for simultaneousradial movement with respect to the disks 20 by an actuator motor 28.Movement enables the data transducers 24 to access different datatracks. The data transducers 24 are positioned using positioninformation supplied by the servo transducer 26 and servo disk surface21. Positional information is derived from signals generated by theservo transducer 26 in reading prerecorded servo bands (indicated at 25)on the servo disk surface 21. Signals generated by the servo transducer26 are passed via line 26a to a preamplifier 30, the output of which ispassed via line 33 to control system 34 which will subsequently bedescribed in greater detail. The function of the control system 34 is toprovide suitable drive current to the actuator motor 28 to move theganged transducers 24 and 26 between servo track positions so as toalign the data transducers 24 with selected data tracks (a seekoperation) or to maintain the transducers 26 and 24 on a selected trackposition during data recording and playback (a track followingoperation). Data signals are passed to or received from the datatransducers 24 along lines 24a by appropriate circuitry which is notdescribed and is not part of this invention. The servo transducer 26,carriage 27, preamplifier 30, control system 34, servo amp 37, power amp39 and actuator 28 (preferably a voice coil motor) form a closed loopservo system. An appropriate input/output control device 32, which isalso no part of the present invention, provides the identity of theservo track locations to be accessed by a digital signal sent along achannel 35 to the control system 34 and interfaces with the datatransducers 24 along the lines 24a. Certain control signals are passedfrom the servo control system 34 to the I/O controller along other linesalso represented schematically by the line 35 in FIG. 1.

Servo Position Encoding

It is necessary for the operation of the described embodiment that theservo bands 25 provide both normal and quadrature positional informationwhereby the incremental (or phase) position of the servo transducer 26with respect to the boundaries of the servo bands 25 can be determined.There are many servo encoding schemes which can be used to provide suchinformation. One such scheme is described in the aforesaid U.S. Pat. No.4,068,269 to Commander et al. which is incorporated by reference hereinin its entirety. FIG. 2 depicts the two cyclical position signals, anormal signal N and a quadrature signal Q, derived from the output ofthe servo transducer 26 as it moves at a uniform radial velocity acrossthe servo surface 21 and bands 25. The two signals, N and Q, areapproximately sinusoidal, 90° out of phase and vary 360° over a set ofservo track pitches. Vertical axes 25' and 25" mark the boundaries of atypical band 25. When the servo transducer 26 is held in a stationaryposition over the disk surface, the values of the normal signal N andquadrature signal Q generated through the transducer 26 are constants.In the prior art, the linear sections of each of the signals N and Q,i.e. sections N of the N signal and Q of the Q signal, were piecewisecombined to create linear position and velocity signals. One importantaspect of the present invention is the combination of the normal signalN and quadrature signal Q so as to identify, with any desiredresolution, the position of the servo transducer 26 with respect to theservo band boundaries (i.e. its phase or incremental position). Thisallows the servo bands 25 to be several times larger, radially, than thedata tracks. It further reduces the sensitivity of the servo system toerrors generated by non-linearities arising in the servo positionencoder portion of the control system. After amplification in the preamp30, the servo position signals detected by head 26 from servo surface 21are passed on line 33 to the servo control system 34.

Servo Control Circuit

FIG. 3 depicts in block diagram form, a preferred embodiment of thecontrol system 34 of FIG. 1 incorporating feedforward and feedbackcontrol. The system 34 comprises a position determination circuit 41, adigital resolver controller ("DRC") 42, a dual gain amplifier ("DGA")circuit 43, a compensator ("COMP") circuit 44, a signal summing junction45, a summer ("SUM") circuit 50, a scheduler circuit 52, a ratemultiplier 54, a feedforward seek drive ("FFSD" or simply "seek drive")circuit 56, a sign inversion ("±1") circuit 58, a voltage controlledoscillator ("VCO") 60, a motor adaptive circuit 62 and an overridecircuit 64. Analog signals are passed on the lines indicated in solidwhile digital signals are passed on channels indicated in dash dot.

Servo Position and Position Error Determination

The position determination circuit 41 accepts the amplified servotransducer 26 signals and generates from those signals, analog normal("N") and quadrature ("Q") transducer position signals. The constructionand operation of the position determination circuit will be dependentupon the position encoding scheme employed and the nature of the signalsrecorded on the servo disk surface 21. The aforesaid U.S. Pat. No.4,068,269 describes a suitable circuit equivalent to the positiondetermination circuit 41 for use with its described encoding scheme. Thenormal and quadrature signals outputted by the system of that patent areapproximately equal to sin y and cos y, respectively, where y is theincremental (phase) position in the radial direction of the servo head26 with respect to (i.e. within) a servo band boundary. Because thenormal and quadrature functions are continuous from servo band to servoband, the position which they define is also continuous. There is noneed to piece together linear portions of each of the two signals.Within each servo band, the normal and quadrature signals uniquelydefine the position of the servo head 26 in a continuous manner makingit possible to servo to any position on the servo surface 21 with equalfacility. The two position signals N and Q are passed to the DRC 42 onlines 71 and 72.

The DRC 42 has suitable circuitry, preferably one or more up/downcounters, to store and output a digital signal indicating a referenceposition, x, the position x being an integer and fractionalrepresentation of servo head 26 radial position with respect to theservo tracks recorded on disk surface 21. The DRC 42 accepts the analognormal signal N and quadrature signal Q from the position determinationcircuit 41 and generates an analog, trigonometric position error signalE_(o) [=-sin (x-y)] representing the phase difference between the actualservo head incremental radial position y, indicated by the servo signalsN and Q (i.e. sin y and cos y, respectively), and the fractional portionof the reference position x. The analog position error signal E_(o) ispassed along line 73 through the dual gain amplifier 43 along line 74 tothe compensator 44 which comprises lead/lag circuitry conventionallyprovided to modify the phase of the position error signal E_(o) toassure stability of the servo loop. The compensated error signal ispassed by the compensator 44 along line 75 through the summing junction45 and along line 36 to the input of the servo amplifier 37, the outputof which is fed on line 38 to the power amplifier 39 supplying currenton line 40 to the actuator motor 28. The servo amplifier 37 and poweramplifier 39 are conventional and designed to accommodate thecharacteristics of the actuator/carriage combination utilized.

Modes of Operation

During a track following operation, the reference position x is held ata constant value (the servo position equivalent to the position of thedata track being accessed) and feedback control of servo head position yis provided by the position error signal E_(o).

Movement of the servo and data heads 26 and 24 to a new position in aseek operation is initiated by the I/O controller 32. The controller 32generates a digital commanded servo position w passed along line 35 tothe servo control system 34 (see FIG. 1). Depending upon theimplementation selected by the user to control the seek operation, w canrepresent the servo head position sought to be reached during the accessoperation or the compliment of that position such that the sum of w plusthe servo head position desired to be accessed, x_(o), equals somepredetermined constant. The preferred embodiment being described usesthe latter mode of operation. The summer circuit 50 adds the referenceposition signal x passed along channel 78 and command position signal w,and passes to the scheduler circuit 52 along channel 80 a digitalsignal, (w+x) in FIG. 3, indicative of the distance between the desiredfinal servo position x_(o) and the presently indicated referenceposition x in the DRC 42. The summer 50 also generates on line 81 alogic level (i.e. high/low) "SIGN" signal, which indicates by its levelthe radial direction (inward or outward) in which the carriage 27 is tomove during the seek. An inverted SIGN signal is passed to an overridecircuit 64 and schedular 52 on line 81. Based upon the value of thesummer circuit output signal (w+x), the scheduler 52 selects and outputsa digital velocity profile signal f(w+x) on channel 82 to the seek drivecircuit 56 and on 82' to the rate multiplier circuit 54. The seek drivecircuit 56 generates in response to the velocity profile signal f(w+x),a feedforward drive current signal which is passed, via the line 84, tothe summing junction 45 and hence, with the positional error signalE_(o) along the line 36 to the servo amplifier 37. Means are provided inthe seek drive circuit 54 to zero the output on line 84 when the systemis in a track following mode and during a seek mode when the servo headsare coasting (i.e. moving with maximum velocity). The seek drive system56 generates a logic level "ON TRACK" signal which is passed on line 85to the dual gain amplifier 43 and, after a slight delay on line 85', tothe motor adaptive circuit 62. The gain applied to the position errorsignal E_(o) is reduced except when the servo control system is in atrack following mode and the ON TRACK signal is at a high level. Thedelayed ON TRACK signal is also available from the seek drive circuit 56to the I/O controller on line 35.

The position error signal E_(o) is also used to control the rate ofchange of the reference position x in the DRC circuit 42 during the seekoperation thereby providing velocity error servo control. The positionerror signal E_(o) is passed via analog line 73 to the sign change (±1)circuit 58. Also forwarded to the sign change circuit 58 are the SIGNsignal outputted by the summer 50 on line 81, an inverted maximumvelocity ("MAX VEL") signal on line 86 form the seek drive circuit 56and FIRST OVERRIDE and SECOND OVERRIDE signals on lines 90 and 91,respectively, from the override circuit 64. With the knowledge of thedirection of carriage motion (indicated by level of SIGN signal 81) andthe sign of the position error signal E_(o) it can be determined whetherthe reference position x leads or lags the servo indicated position y.The purpose of the sign change circuit is to assure that a positionerror voltage of the proper sign is passed to the VCO 60 so as toincrease the rate of change of the reference position x when thatposition lags the servo position y, or decrease the rate of change atthe reference position x when that position leads the reference positiony. The sign change circuit 58 of the preferred embodiment beingdescribed also includes an anticipate subcircuit to scale the voltagelevel of the position error signal E_(o) to account for inductance inthe actuator motor 28 so as to prevent the carriage 27 from overshootingits model deceleration curve profile during short seek operations.

The output of the sign change circuit 58 is applied via the channel 87to the voltage control oscillator 60 which is of a conventional design.The voltage control oscillator 60 outputs on line 88 a clock (high/low)signal, the frequency of which is controlled about a nominal frequencyby the voltage level of the sign change circuit 58 output signalprovided on line 87. A positive voltage from the sign change circuit 58increases the frequency of the clock signal outputted by the VCO on line88 while a negative voltage decreases that frequency. The magnitude ofthe signal from the sign change circuit 58 controls the extent to whichthe VCO clock signal frequency is increased or decreased.

The clock signal from the VCO circuit 60 may be passed directly to therate multiplier 54 or, preferably as in the embodiment being described,to a motor adaptive circuit 62 located between the output of the VCOcircuit 60 and the clock input of the rate multiplier circuit 54. Themotor adaptive circuit 62, if provided, modifies the frequency of theVCO clock signal in response to actual performance of the servo systemduring the acceleration portion of a longer seek operation and passesthe modified signal on channel 89 to the rate multiplier circuit 54during the deceleration portion of the seek. The rate multiplier circuit54 scales the clock pulse signal it receives on the channel 89 by thevalue of the scheduled velocity signal f(w+x) it receives from thescheduler circuit 52 via the channel 82' and provides yet a differentclock pulse signal via the channel 93 to the DRC circuit 42 toincrementally change the reference position x to program the change inactual position y of the servo head 26.

During a seek operation, the primary portion of the drive signal passedto the servo amp 37 is generated by the seek drive circuit 56. The seekdrive circuit 56 generates in response to the value of the velocityprofile signal, a drive signal suitable for a model actuator operatingto its nominal design specifications. However, control is still providedthrough feedback of the position error signal E_(o) at the summingjunction 45.

During the acceleration portion of a seek operation, the rate multiplier54 will update the reference position x in the DRC 42 sufficientlyrapidly to cause the reference position x to overshoot the actual servoposition y by more than a servo band (i.e. band skip). To prevent thisfrom occurring, an override circuit 64 is provided. In the preferredembodiment being described, the override circuit 64 outputs a logiclevel signal on line 92 which controls the direction in which thereference position x is incremented (i.e. increased or decreased) by therate multiplier 54. The override circuit 64 includes suitable circuitryto monitor the magnitude of the position error E_(o) carried to it onthe line 73. When the position error signal E_(o) exceeds a certainabsolute value, the override circuit changes the logic signal on line 92reversing the direction of incrementation of the reference position xand preventing the reference position x from further overshooting theactual servo position y. When the magnitude of the position error signalE_(o) is reduced to acceptable levels, the level of the signal outputtedby the override circuit 64 on the line 92 is returned to that whichallows the reference position x to again lead the servo indicatedposition y.

Control System Components

Each of the major circuits 42 through 64 will now be described ingreater detail.

Position Detection Circuit

U.S. Pat. No. 4,068,269 to Commander et al., previously incorporated byreference, is referred to for a description of a suitable data diskencoding scheme and a position detection circuit 41, identified thereinas a "position error detect circuit 25". It will be appreciated thatthis represents one of many known servo encoding schemes for providingboth normal and quadrature position signals which may be employed withthe present invention. It is not intended that the present invention beconstrued to be limited for use solely with that position detectionembodiment or servo encoding scheme.

Digital Resolver Controller 42

FIGS. 4a and 4b depict the components of the digital resolver controller("DRC") 42 of FIG. 3. The DRC 42 comprises two major subcircuits: areference position indicator 160 indicated in FIG. 4a and a positionerror circuit 161 indicated in FIG. 's 4a and 4b. The reference positionindicator 160 stores the reference position x and generates a 16-bitdigital reference position signal (referred to as x_(i)). The depictedcircuit 160 is formed by four cascaded four-bit digital counters 181through 184 (type LS 169) providing sixteen bits for reference positionx identification. Greater or fewer bits may be provided and the bits mayorganize as desired for data track and servo track representation. Inthe embodiment being described, the eight bits provided by the uppercounters 181 and 182 identify integer servo bands while the eight bitsprovided by counters 183 and 184 provide fractional resolution of eachindividual band, down to approximately 1.4° (or 2.5×10⁻² radians). Allsixteen bits x_(i) are made available to the summer circuit 50 ondigital channel 78 (see FIG. 3). In addition, the least significanteight bits (i.e. those of counters 183 and 184), hereinafter referred toas x are made available to first and second function generators 162 and164, respectively, of the position error circuit 161 via the digitalchannel 186.

The position error circuit 161 includes the two function generators(programmable read only memories type TBP 18S22) 162 and 164, as well asfirst and second multiplying digital to analog converters 166 and 168(type AD 7523), respectively, and five, identical operational amplifiers172, 174, 176, 178 and 180 (type LF 347).

As the name would imply, the position error circuit 161 generates aposition error signal representing the phase difference between theactual servo position y, as indicated by the normal and quadratureposition signals N and Q, and x, the fractional portion of the referenceposition x. Several implementations would be suitable. For high speedapplications, a trigonometric relationship is preferred. The positionerror signal E_(o) is represented in this embodiment by therelationship:

    E.sub.o =-sin x cos y +cos x sin y =-sin (x-y)

but the relationship:

    E.sub.o =sin y -cos y tan x

might also be used where: y is the actual (phase) position of the servohead 26, sin y and cos y are the outputs of the position detectioncircuit 41 and x is the fractional or phase portion of the referenceposition x (i.e. the eight least significant bits, x₀ through x₇). Ineither case, if x equals y the relationship goes to zero.

The first PROM 162 outputs in response to the digital reference positionbits x₀ through x₇, a digital approximation of the trigonometricfunction cos x. The second PROM 164 similarly outputs a digitalapproximation of -sin x. The normal position signal N supplied by theposition detection circuit 41 is presumed to be sin y (the circuit 41should be so implemented), which is fed via the line 71 into thereference input of the first multiplying digital to analog converter 166while an eight bit digital representation of the cos x is fed from thegenerator 162 into the digital inputs of the converter 166. The outputof the converter 166 is passed to a pair of the operational amplifiers172 and 174 which, with the indicated circuit elements, convert thecurrents outputted by lines B and C to produce at the output ofoperational amplifier 172, an analog voltage proportional to the product(cos x) (sin y). Similarly, the quadrature servo position signal Q (cosy) is fed on line 72 to the reference input of the second multiplyingdigital to analog converter 168 while an eight bit digital approximationof the -sin x from generator 164 is fed into the digital inputs of thatdevice. Associated with the second converter 168 is a similar network ofcircuit elements including the two operational amplifiers 176 and 178,the output of amplifier 176 being an analog voltage output proportionalto the product -(sin x) (cos y). Schottkey diodes 179 are provided toprotect the converters 166 and 168 from transients. The outputs of thetwo amplifiers 172 and 176 are fed into the negative input of theoperational amplifier 180, the positive input of which is tied toground. The amplifier 180 inverts as well as sums the outputs from amps172 and 176 providing an analog signal proportional to the value -sin(x-y), i.e. E_(o). As is indicated in FIG. 3, the position error signalE_(o) is carried via line 73 to the dual gain amp circuit 43, seek drivecircuit 56, override circuit 64 and sign change circuit 58. As isfurther indicated in FIG. 4b, for convenience the outputs of theamplifiers 172 and 176 are also carried directly on lines 73' and 73",respectively, to the seek drive circuit so that the signal -E_(o) mightbe used in that form. If desired, suitable circuitry can be provided inthe seek drive circuit 56 to invert the position error signal, E_(o),passed on line 73 for use therein.

During the track following mode, the rate multiplier 54 circuit does notoutput a signal to the counters 181-184 and thus holds the referenceposition x at a constant value representing the servo positionequivalent of the data track desired to be followed. The position errorsignal E_(o) from the DRC 42, as modified by the dual gain amplifier 43and compensator circuit 44, is passed to the servo amplifier 37controlling the power amplifier 39 supplying current to the voicecontrol motor 28 holding the ganged transducers 24 and 26 in a fixedposition .

Dual Gain Amplifier 43

An exemplary dual gain amplifier 43 is depicted in FIG. 5 and comprisesan operational amplifier 190 (type LF 347) receiving the position errorsignal on line 73 at its negative input, a field effect transistorswitch 192 (type IH 5011), a TTL buffer 194 (7407) and variousresistors. An ON TRACK signal is passed from the seek drive circuit 56along the line 85 through the TTL buffer 194 to the gate 192' of thefield effect transistor switch 192 controlling its operation. During aseek mode of operation, the ON TRACK signal is low and the switch 192 isclosed whereby the gain of the position error signal is reduced. Whenthe ON TRACK signal is high (occurring in a track following mode), theswitch 192 is open boosting the position error signal gain. The positionerror signal E_(o) is applied to the servo amp 37 during seek operationsinstead of being eliminated entirely as it is believed to provide asmoothing action when the carriage is approaching the desired servoposition during the seek and the servo control system is transitioningfrom a seek to a track following mode.

One skilled in the art will appreciate that the depicted embodiment isnot so sensitive to deterioration of the servo position signals, N andQ, as the prior art references cited. However, it will further beappreciated that the accuracy of the inventive embodiment beingdescribed in the track following mode will only be as good as thecorrelation between the functional representations of the referenceposition (i.e. sin x and cos x) and the corresponding values of theservo position signals N and Q for the same servo positions. The servoencoding scheme and apparatus described in the aforesaid U.S. Pat. No.4,068,269 to Commander et al. is biased to maximize the linearity of theservo position signals N and Q. The described embodiment using such aservo encoding and decoding scheme will be most accurate at phasepositions separated by 45° (i.e. 0°, 45°, 90°, etc.). Thus in thedepicted embodiment, up to eight data tracks may be positionedcorresponding to each servo band and followed with equivalent accuracywhereas in the prior art only four data tracks could be provided foreach servo band. It will further be appreciated that the resolution ofthe DRC can be changed by varying the number of digital bits providedfor reference position indication and/or the number of bits allocatedfor reference position phase indication (i.e. fractional portion ofreference position x). Further, any values can be included in thefunction generators (PROMs 162 and 164) of the DRC 42. The trackfollowing ability of the system can be made as accurate as desired bymodifying the stored functions to correspond to the servo encoder valuesbeing generated.

Movement of the ganged transducers 24 and 26 from their existingreference position to a new reference position x₀ and thus to a newcorresponding data track is accomplished by means of a commandedposition signal w supplied from the host computer (not depicted) throughI/O controller 32 (see FIG. 1). Control during a seek operation could beaccomplished by subtracting the existing reference position x from thecontrol position w and controlling to the null condition (i.e. w-x=0).However, the preferred embodiment of the invention utilizes parallelcontrol logic for increased speed and is most conveniently implementedto provide control on the condition w+x₀ =constant where, for thedescribed embodiment, the constant is 2¹⁶. The 16-bit commanded positionw is selected to drive x to the desired servo position x₀.

Summer Circuit

The summer circuit 50 is depicted in FIG. 6 and implemented to output adigital signal x+w. The summer circuit 50 comprises cascaded 4-bitadders (type LS 283) 220, 222, 224, and 226. The first 220 accepts thefour least significant bits of the digital command position signal w online 35 together with the four least significant bits of the positionreference signal x on the channel 78 summing those two inputs. A onecarry, if present, is passed on line 221 to the second counter 222 whichaccepts the next four significant digits from the command positionsignal w and reference position signal x. Line 223 is provided to pass aone carry, if present, to the third counter 224 which sums the next fourmost significant digits of the two position signals w and x and passes aone carry, if present, to the fourth counter 226 on the line 225. Thefourth counter 226 sums the four most significant digits of the twoposition signals w and x. A one carry provided from counter 226 ispassed along line 81 and is hereinafter referred to as the SIGN signal.The SIGN signal indicates by its level, the direction the carriage 27must move during the seek. The counters 220, 222, 224, and 226collectively output the 16 bit summed position signal w+x on line 80.This signal represents distance to go between the current referenceposition x and the desired position x₀. At null, the 16 bit outputs ofthe counters 220, 222, 224, and 226 on line 80 are all zero with a highlevel SIGN signal on line 81. NAND gate 250 provides an inverted SIGNsignal on channel 81. When w+x is greater than 2¹⁶, the SIGN signal isat a high level and the 16-bit summer outputs on line 80 indicate theerror. When w+x is less than 2¹⁶, the SIGN signal on line 81 is at a lowlevel (i.e. no carry) and the summer outputs on line 80 are equal to thecompliment of the error minus 1.

Schedular

The outputs of the summer circuit 50 are fed on line 80 into theschedular circuit 52, depicted in detail in FIG. 7. The schedular 52includes a comparator (type 8160) 240, function generators (EPROM type2716) 242 and 244, a first latch 246 (type LS 374) and a D-typeflip/flop 248. Each of the generators 242 and 244 is programmed toprovide an appropriate velocity profile signal f(w+x) in response to thesummer output signal w+x. If desired, a microprocessor could be providedto form the functions of the summer 50, schedular 52 and other of theelements of this embodiment. However, the use of the depicted parallellogic circuit with the look-up table capabilities of the read onlymemories has two advantages over the use of a microprocessor. The firstis that the parallel logic circuits embodiment is faster. The second isthat the schedular velocity profile signal f(w+x) can be any arbitraryfunction since it is held in a table in the memories 242 and 244. Thisis important as velocity control of the actuator 28 is performed in thepresent embodiment as a function of position and displacement. With"bang-bang" control, optimum deceleration is given by the equation:

    2.sup.16 -(w+x)=v.sub.1 t[v/v.sub.1 +ln (1-v/v.sub.1)]

where the left side of the equation is distance to go and v is servohead velocity, v₁ is the self-limiting head velocity (=V_(max) /K) and tis the mechanical time constant (=MR/K²). K is the motor transductionconstant, V_(max) is the maximum voltage applied to the voice coilmotor, M is the carriage mass and R is the coil resistance of the voicecoil motor. This defines x as a function of v but what is needed is v asa function of x, which is not as obvious. The values of v as a functionof x (or w+x) can, however, be readily determined from this equation andstored in the look-up table of the memories 242 and 244. The values usedfor K, V, M and R are worst case values (to provide greater than thefastest possible response) so that all drives would be able to remainunder control. Two memories 242 and 244 are provided simply to accept asufficient range of summer signal outputs. The first memory 242 acceptsthe ten most significant bits of the sixteen bit summer circuit outputsignal w+x while the second memory 244 accepts the ten least significantbits of that signal. Control over the memories 242 and 244 is maintainedby the comparator 240 which receives the six most significant digits ofthe summer output signal w+x as well as the inverse of the SIGN signalon the line 81. The comparator 240 determines from the six mostsignificant bits and the inverse SIGN signal whether the remainingdistance to go is greater or less than 10 bits and activates theappropriate read only memory 242 or 244, respectively, by logic signalon line 243. The signal sent to memory 242 is inverted by inverter 252and passed to the second memory 244. In the embodiment being described,the first memory 242 controls during longer seeks (i.e. when difference|2¹⁶ -|w+x|| greater than or equal to 2¹⁰ (about 5 milliseconds)) whilethe second memory 244 controls for shorter seeks and during finalapproach of longer seeks.

A nine-bit digital velocity profile signal f(w+x) is fed by a selectedone of the two read only memories 242 and 244 to the seek drive 56 onchannel 82 and to the rate multiplier 54 on channel 82', the latter byway of the latch 246 and flip/flop 248. The velocity profile signalcomprises seven bits outputted by whichever memory 242 or 244 isactivated by the comparator 240 plus the inverse of the most significantbit outputted by the first memory 242 and the least significant bitoutputted by the second memory 244. The most significant bit of memory242 is also passed, after inversion by gate 254, to the motor adaptivecircuit 62 on line 83. The velocity profile signal f(w+x) is clocked outof the latch 246 and flip-flop 248 to the rate multiplier 54 by a clocksignal generated by the motor adaptive circuit 62 and passed on line 89.The latch 246 also accepts a signal on line 91 from the override circuit64 (see FIG. 3) which produces a maximum velocity profile signal,f(w+x)_(max), loaded into the rate multiplier 54 when an overridecondition is reached.

Seek Drive

The major components of the seek drive circuit 56 are depicted in FIG. 8and comprise a reference voltage circuit 290, a deceleration functiongenerator (type 2716 PROM) 292, a multiplying digital to analogconverter (type AD-5723) 294 with associated operational amplifier 296and second operational amplifier 298 (both type LF 347). The seek drivecircuit 56 produces an anticipated linear motor drive signal during aseek operation for feedforward control. This signal is passed from theamplifier 298 along the line 84 to the summing junction 45 and hence tothe servo amplifier 37 and power amplifier 39 so as to control themotion of the carriage 27 during the seek operation. The referencecurrent source circuit 290 produces an analog reference voltage which ismodified by elements 312 and 314 and passed on line 291. The modifiedreference voltage is either positive or negative depending upon thedirection of the desired acceleration (or deceleration).

The acceleration and deceleration of the carriage 27 during the seekoperation and, therefore, the current used to drive the actuator 28, isa function of the scheduled velocity f(w+x). The following accelerationfunctions are used:

    a=0 for v=v.sub.max (coasting);

    a=(1/t)(v.sub.1 -v) for f(w+x)=v.sub.1 t[v/v.sub.1 +ln (1-v/v.sub.1)] and (decelerating);

and

    a=kv for v=k[f(w+x)] (approaching on track)

where a is acceleration, and v, v₁ and t are as previously defined. Thefunction generator 292, which is addressed by the velocity profilefunction f(w+x), is used to store the deceleration values to produce adigital deceleration function. A zero output is produced by generator292 during the track following mode, in response to a zero velocityprofile function (f(w+x)=0). The function generator 292 also generatesan ON TRACK signal for this condition. In the digital to analogconvertor 294, a product of the digital deceleration function outputtedby the generator 292 and the modified analog reference voltage on line291 is produced. This product is the feedforward drive signal outputtedby amplifier 298.

By providing the feedforward drive signal, feedback control using theposition error signal, E_(o), can be reduced. During seek operations, ingain of the position error signal E_(o) is reduced in the dual gainamplifier 43 in response to the level of the ON TRACK signal generatedby the function generator 292. Providing feedforward control alsoreduces the operating range requirements on the voltage controloscillator circuit 60 allowing a simple single integrated circuit to beused, as will be later described.

The components of an exemplary reference current circuit 290 comprise afirst switch 300, a toggling amplifier 304 and a second switch 306passing the initial reference voltage during acceleration anddeceleration modes. The first switch 300 is an FET type IH 5011 and iscontrolled by the SIGN signal outputted by the summer 50 on the line 81which is applied to the gate 300' of the transistor forming the switch300. The resistor 302' associated with the negative voltage source 302(-15 V) has a magnitude one-half that of the resistor 301' associatedwith the positive voltage surface 301 (+15 V). Thus, when the switch 300is open (SIGN signal is high) a positive current produced by the source301 and resistor 301' is fed to the inverting input of togglingamplifier 304. When the switch 300 is closed (SIGN signal is low) anegative current is produced. The switch 300 signal selects theappropriate current sign (positive or negative) for decelerating thecarriage 27 during the seek operation. The current outputted by theswitch 300 and resistor 301' is summed at the locations 303 with acurrent proportional to the position error signal outputted directly byamps 172 and 176 on lines 73' and 73" (see FIG. 4b) and the compositecurrent is fed into the amplifier 307 the output of which is clamped byback to back Zener diodes 308 and 309 feeding back to the invertinginput of the amplifier 307. When a seek operation is initiated, theposition error E_(o) becomes very large initially. The large E_(o)produces a current which overrides the reference current outputted fromthe switch 300 and resistor 301'. The output of the amplifier 306reverses in sign (toggles) so as to output a voltage with an appropriatesign for acceleration. When the actual velocity approaches the scheduledvelocity, the position error approaches zero. The current produced at303 by the components of the position error signal is no longersufficient to override the reference current produced by switch 300 andresistor 301' and the output of toggling amplifier 304 switches,outputting a voltage with a sign suitable for deceleration. Positivefeedback to amplifier 307 also changes the level at which the positionerror can cause the switch 304 to toggle, thus preventing retoggling bylarge position errors during deceleration. The output from the togglingamplifier 304 is passed to yet another switch 306 (FET type IH 5011) topass an initial reference voltage during acceleration and deceleratonmodes of the seek and to prevent the passage of a reference voltageduring a coasting portion of the seek operation or during a trackfollowing operation. This is accomplished by NAND logic gates 316 and318. The first NAND gate 316 receives as one of its inputs, an outputfrom the override circuit 64, which is high when the position error iswithin satisfactory magnitude limits and low when the error exceedsthose limits. It also receives the maximum velocity ("MAX VEL") controlsignal outputted as the most significant bit of the eight-bit functiongenerated by the function generator 292. This signal is high during themaximum velocity period (i.e. coast segment) of the seek operation, andlow otherwise. The second NAND gate 318 accepts the output of the firstNAND gate 316 and the least significant bit outputted directly by thefunction generator 292. This bit signal is high during a seek mode andlow during a track following mode. After inversion by the inverter 320and 322, this signal is referred to as the ON TRACK signal and is passedto the I/O controller 32 on line 35 to indicate that track followingoperations (i.e. read or write) may be commenced, and to the dual gainamplifier 43 on line 85 for controlling the gain of the position errorsignal E_(o) passed by that circuit. The output of the switch 306 iscombined with the output of a time constant circuit 312 at the point 311and fed to the negative input of an operational amplifier 314. Thevariable resistor 312a, resistors 312b, 312c, and 312d and the capacitor312e of the time constant circuit 312 provide an output adjusting theinitial reference voltage passing from the switch 306 to account formotor inductance. The amplifier 314 outputs the modified referencevoltage on line 291 to the multiplying input of the digital to analogconvertor 294.

Another operation which must necessarily be performed during the seekoperation is the updating of the servo reference position x. This isaccomplished through the position error signal E_(o) outputted by theDRC 42, the sign change (±1) circuit 58, the voltage controlledoscillator circuit 60, motor adaptor circuit 62, rate multiplier 54 andoverride circuit 64.

Sign Change Circuit

The function of the sign change circuit 58 is to multiply the positionerror signal E_(o) by an appropriate sign (i.e. ±1) in order that themagnitude of the position error be passed to the voltage controloscillator with the proper sign. In addition, the sign change circuit 58prevents the servo system from overshooting its desired profile whenswitching from acceleration to deceleration. The components of the signchange circuit 58 are depicted in FIG. 9. Sign change is accomplished bymeans of subcircuit 325 including an operational amplifier 326 (type347) and a transistor switch 327 (type IH5011). The switch 327 iscontrolled by a pair of buffer gates 328 and 329 which together act asan equivalent AND gate. The SIGN signal generated by the summer circuit50 is passed on line 81 through the first buffer 328. Another signal ispassed from one of a pair of analog comparators in the override circuit64 along the line 90 to the second buffer 329. The level of this lattersignal is high except when the position error E_(o) exceeds the positiveoverride limit. The position error E_(o) outputted by the DRC 42 ispassed via the line 73 to the subcircuit 325. The sign of the positionerror E_(o) will indicate overshoot or undershoot (i.e. lead or lag) ofthe actual position y by the reference position x, depending upon thedirection in which the carriage 27 is being moved. As the voltagecontrolled oscillator 60 responds to a positive voltage input byincreasing the frequency of its output signal and a negative voltageinput by decreasing the frequency of its output signal, it is necessaryto control both the magnitude and the sign of the positional errorsignal sent to that circuit. When the switch 327 is open, the amplifier326 becomes a unity gain following amplifier and the positional error ispassed unchanged in sign. If the transistor switch 327 is closed, thenthe position error signal E_(o) is not passed to the positive input ofamplifier 326 and feedback makes it an inverting unity gain amplifierreversing the sign of the position error signal.

An anticipate subcircuit 330 is formed by operational amplifier circuit332, a logic (NAND) gate 334, capacitor 336 and resistor network 337.Inputs to the NAND gate 334 are an inverted maximum velocity signal (MAXVEL) outputted by inverting gate 293 of the seek drive circuit 56 (seeFIG. 8) and an output from the override circuit on line 91 which is highwhen the servo system is in the override condition (i.e. during initialcarriage 27 acceleration). During initial acceleration before maximumvelocity is reached, the servo is in the override condition and theoutput of the NAND gate 334 is low. This clamps the capacitor 336 tozero causing the resistor network 337 to output some voltage lower thana nominal voltage. As the carriage 27 approaches nominal velocity, theoverride condition disappears and the output of the NAND gate 334 goeshigh causing the voltage outputted by the resistor network 337 to rise,thereby increasing the voltage level outputted by the amplifier 332 tothe VCO 60. The effect is to lower the anticipated deceleration curveduring acceleration, beginning deceleration earlier so as not toovershoot the nominal deceleration curve. For long seeks this is not aproblem so the anticipate function is locked out by a low level signalon line 86. Gate 335 (type LS 14) buffers the NAND gate 334 output.Resistor network 331 is provided to suitably bias the nominal voltageoutputted by the second amplifier 332.

Voltage Controlled Oscillator ("VCO")

Output from the second amplifier 332 is passed on line 87 to the voltagecontrolled oscillator circuit 60 depicted in FIG. 10. The VCO is of aconventional design and comprises a type LS 325 oscillator 338 andassociated circuit elements indicated generally by the notation 339. Thenetwork 339 and the output of the anticipate circuit 330 of FIG. 9effect the bias of the oscillator 338. The oscillator 338 outputs online 88 a pulsed (high level/low level) signal the frequency of which iscontrolled by the voltage from the second amplifier 332 and is of anominal value when that voltage is zero.

Motor Adaptive Circuit

The output of the voltage controlled oscillator 60 is carried to themotor adaptive circuit 62. In the depicted servo system, the scaling ofthe velocity profile for deceleration of the carriage 27 is governed, inpart, by the voltage controlled oscillator 60 which drives the ratemultiplier 54. By adjusting the frequency output of the VCO 60, theservo can be adapted to accommodate a wide range of motor and driveamplifier constants. The purpose of the motor adaptive circuit is tomake the servo control system self-adapting by measuring theacceleration performance of the power amplifier 39/voice control motor28 combination during the acceleration portion of a seek and adjustingthe servo control during deceleration by appropriate scaling. The motoradaptive circuit 62 is not limited to the preferred embodiment beingdescribed or to position control servos generally, but is believedadaptable to any servo system using a changing frequency signal, such ascan be outputted by a voltage controlled oscillator. It may also bemodified for use with a system employing a signal with another variablecharacteristic (e.g. amplitude) for feedback control.

The major elements of the motor adaptive circuit 62 are depicted in FIG.11 and include a first latch 340, a function generator 342, a secondlatch 344, a rate multiplier 346 and a counter 348. Four bits of thereference position x (i.e. x₅ -x₈) are fed to the first latch 340.Operation of the latch 340 is controlled by the ON TRACK signal passedfrom the seek drive circuit 56 on line 85. When the servo control systemfinishes a seek operation, the ON TRACK signal changes to a high leveland the latch stores the on track values (i.e. the four identified bitsof the reference position x). The four bits stored in the latch 340, aswell as current values of those same four bits of the reference positionx are carried to the function generator 342 which is another read onlymemory (EPROM type 2716) which continuously compares the latched valueswith the present indicated values. Based upon the difference between thetwo sets of values, the generator 342 repeatedly provides a four-bitdigital function to the second latch 344 along lines 343. This functionis initially high during the beginning of acceleration but drops as theacceleration period progresses and the carriage 27 begins to move. Thefunction programmed into the function generator 342 is determined for afixed period of acceleration. That period should be less than themaximum acceleration (about 8 milliseconds in the embodiment beingdescribed) and should correspond to the time needed to travel thedistance of the selected reference position bits (i.e. x₅ -x₈) beingcompared (about 5 milliseconds in this embodiment). The output of thevoltage controlled oscillator 60 is not affected unless the accelerationperiod is at least as long as the predetermined period upon which thefunctions in the function generator 342 are based (i.e. 5 milliseconds).The second latch 344 is set by an appropriate signal generated tocoincide with the end of the predetermined acceleration period uponwhich the function is based.

In the depicted embodiment this signal is generated by the counter 348and associated NAND gates 350a, 350b, 351a and 351b. The logic elements350a and 350b activate the counter 348 at the beginning of the seekoperation if the most significant bit of f(w+x) goes high. The serialNAND gates 350a and 350b act as an equivalent AND gate. The inputs tothe first NAND gate 350a are the ON TRACK signal on line 85 from theseek drive circuit 56 and the inverse of the most significant bitoutputted by the function generator 243 of the schedular circuit 52 online 83. The signal reaching NAND gate 350a on line 83 goes high onlywhen the first generator 242 indicates that the seek operation willrequire at least five milliseconds time. The ON TRACK signal on line 85goes low when the new seek is initiated. This is delayed to gate 350a bythe RC time constant. Thus the output of gate 350a momentarily goes lowwhen a sufficiently long seek is initiated. The output of NAND gate 350bwill pulse high when the seek operation is to require at least 5milliseconds duration. This resets counter 348 to count duringacceleration. A suitable clock source is provided in the presentembodiment by an oscillator made up of NAND gates 351a and 351b andassociated circuit elements, indicated generally as 351c. After beingcounted down, the output signal of the counter 348 passed to the latch344 on line 352 sets the latch 344, storing the last function passed toit by the function generator 342. The function outputted by thegenerator 342 is related to the actual distance travelled by thecarriage 27 during the predetermined acceleration period. It is directlyproportional to acceleration. The function is used to scale thedeceleration of the carriage 27 by programming the rate multiplier 346which divides down the frequency of the signal outputted by the VCO 60before it is passed to the velocity programming rate multiplier circuit54. If the acceleration period of a seek operation is not as long as thepredetermined acceleration period on which the motor adaptor circuit 62is based, the second latch 344 is not set and the rate multiplier 346uses the last stored value to adjust the VCO output. Without a motoradaptive circuit 62, the voltage controlled oscillator 60 is settypically for the slowest or nominal motor carriage configuration. Withthe motor adaptive circuit 62, the VCO is selected for a better than thebest (i.e. the fastest) possible motor/carriage combination and the VCOoutput reduced by the rate multiplier 346. This prevents overflow of thelatch 342. A reduction of up to 50% carriage acceleration will beautomatically accommodated by the circuit.

The relationship between the VCO signal passed into the motor adaptorcircuit 62 on the line 88 and the output of the motor adaptor circuit onthe line 89 is given by the following equation:

    f.sub.mca =f.sub.vco (M/N)

where N is the constant for the rate multiplier 346, M is provided bythe function generator 342, f_(vco) is the signal outputted by VCOcircuit 60 on line 88 and f_(mca) is the signal outputted on line 89after acceleration. The SIGN signal generated by the summer circuit 50is passed on line 81 to the function generator 342 to indicate thedirection of travel of carriage 27 and which of the two values, initialreference position stored in the latch 340 or current reference positionpassed directly to the generator 342, is to be subtracted from theother. The generator 342 is concerned only with the absolute magnitudeof the distance travelled.

Rate Multiplier

The rate multiplier circuit 54 of the preferred embodiment is depictedin FIG. 12 and comprises, as the name implies, a rate multiplier formedin the depicted embodiment by first and second 6-bit (type 7497) ratemultipliers 281 and 282. The first multiplier 281 accepts the threeleast significant bits of the velocity profile signal f(w+x), i.e., thetwo least significant bits outputted by the latch 246 and the output ofthe one bit latch 248 of the scheduler 52. The second multiplier 282accepts the six most significant bits of the velocity profile functionpassed from the latch 246. The two multipliers 281 and 282 are gangedfor simultaneous action as a single multiplier. A clock pulse signaloutputted by the motor adaptive circuit 62 on the line 89 (or VCO ifmotor adaptor circuit is not provided) is fed to the clock inputs of themultipliers 281 and 282. The frequency of the signal outputted on theline 89 by the motor adaptive circuit 62 is a function of both theposition error signal E_(o) and the actual performance of the servosystem. The second multiplier 282 outputs a pulsed signal on line 93carried to the reference position indicator 160 of the DRC 42. The ratemultiplier signal is used to increment bit-by-bit the ganged, up/downcounters 181-184 of the reference position indicator 160. In this way,during the seek operation, the reference position x is driven to followthe actual servo position y for acceleration and to program the servoposition (or velocity) for deceleration.

Override Circuit

The override circuit 64 monitors the position error E_(o) and outputsappropriate control signals when that error becomes exceedingly large.The override circuit 64 is depicted in detail in FIG. 13 and comprises acomparator subcircuit 390, a first latch 392, a counter control logiccircuit 394, and a second latch 396. The purpose of the comparator 390is to constantly monitor the magnitude of the position error signalE_(o) on line 73 against predetermined limits. An analog comparatorcircuit 390 is provided and comprises a pair of voltage comparators 398and 400 and a NAND logic gate 402. The output of both comparators 398and 400 is high when the position error E_(o) is within acceptablelimits. Depending upon the sign of the position error, E_(o), one of thecomparators 398 or 400 will go low if the magnitude of the positionerror signal exceeds a predetermined value. The output of the firstcomparator 398 monitoring maximum (positive) position error is carriedas a control signal to the sign change circuit 58 along the line 90. Theoutputs of both comparators 398 and 400 are fed to the inputs of theNAND gate 402 the output of which is passed to the first latch 392. Theoutput of the NAND gate 402 is low when the position error is withinbounds and high when it exceeds a positive or negative limit. The outputof the NAND gate 402 is passed through latch 392 to counter controllogic circuit 394 and along line 91 to logic in the sign change circuit58 and latch 246 in the schedular circuit 52 previously described. Aninverted signal is passed from the inverter 394d to the seek drivecircuit on channel 91. Output of the override circuit 64 is synchronizedby clocking the latch 392 using the output of the motor adaptive circuit62 passed along the line 89 (or output of the VCO 60 or line 88 if amotor adaptive circuit is not supplied).

The counter control logic 394 outputs a signal used to control theoperation of the reference position indicator 160 in the DRC 42. In theembodiment depicted, the cascaded counters 181-184 forming the referenceposition indicator 160 are incremented by means of a clock signaloutputted by the rate multiplier 54 along the line 83. The direction inwhich the counters 181-183 of the reference position indicator 160 areincremented is controlled by the level of the signal outputted by thecounter control logic circuit 394 and latch 396. When the position errorE_(o) is within predetermined tolerances, the level of the signal passedfrom the latch 396 on the line 92 is now at an appropriate level toincrement the counters 181-185 in the reference position indicator 160in the direction in which the servo head 26 is to be moving as indicatedby the sign bit produced by the summer circuit 50 and outputted on line81. When the magnitude of the position error E_(o) exceeds thepredetermined limits embodied in the comparator circuit 390, the levelof the signal outputted by the counter control logic 394 and latch 396is reversed causing the counter to increment in the opposite direction.

While a preferred embodiment of the invention has been described, otherembodiments are possible and may be preferred to balance diminishedperformance capability with diminished costs. For example, the motoradaptive circuit may be dispensed with if variation in the motorparameters is small. In such case, the control system would be modeledto the performance of a worst case positioning system whereas thepreferred embodiment is modeled to a best case (i.e. fastest response)positionig system. As was previously indicated, a microprocessor couldbe incorporated to perform the function of the summer and schedularcircuits and perhaps the functions of some of the other circuits of thecontrol system (i.e. the DRC). It is believed, however, that inexpensivemicroprocessors commercially available at the present time will notprovide the response time provided by the described preferredembodiment. If desired, the feedforward seek drive circuit may beeliminated and the servo head control can be provided during seekoperations by means of a variable gain amplifier. The amplifier wouldoutput a signal proportional to the position error signal E_(o), thegain of which would be controlled by the magnitude of velocity profilesignal f(w+x). In each of these other embodiments, servo control isstill accomplished by means of the position error signal E_(o). Thisavoids the necessity of differentiating that signal to provide controlduring a seek operation. Moreover, while the present invention has beendescribed with respect to controlling the positioning of gangedtransducers for a magnetic data storage apparatus, one skilled in theart will appreciate that the invention could be applied to other datastorage forms such as elongated webs and drums, thus, the invention isnot necessarily limited to controlling a strictly linear motion but maybe used to control motion along a single path. Moreover, the inventionmay be used with other types of storage devices, particularly opticaldata storage disks, and may find application outside the data storagefield.

What is claimed is:
 1. In a positioning system for moving a movablemember along a defined path of movement from an initial position to afinal position and including the movable member having the defined pathof movement, means for dividing the path of movement into a series ofcontiguous segments, actuator means coupled to the movable member formoving said movable member, a control system comprising:referenceposition means for storing a reference position of the movable memberwith respect to the path of movement, the stored reference positionbeing variable, indicating an approximate position of the movable memberwith respect to the segments along the path of movement including anapproximate incremental position with respect to a segment and initiallyindicating said initial position; position means for generating anindication of actual incremental position of the movable member withrespect to a segment; controller means responsive to said position meansand to said reference position means for generating a position errorvalue having a magnitude indicating a difference between saidapproximate incremental position and said actual incremental position;means for controlling said actuator means in response to said positionerror value; and updating means responsive to a command indicating afinal position of the member for incrementing the stored referenceposition from said initial position to said final position.
 2. Thepositioning system of claim 1 wherein said updating meanscomprises:means for generating a scaling function having a magnitudediminishing as the stored reference position approaches the finalposition; and incrementing means for incrementing the stored referenceposition towards said final position at an incrementation rate relatedto the magnitude of said scaling function.
 3. The positioning system ofclaim 2 wherein said updating means further comprises:feedback means forvarying the incrementation rate of the incrementing means in response tosaid position error value.
 4. The positioning system of claim 3 whereinthe updating means further comprises:override means for continuouslycomparing the magnitude of the position error value with a predeterminedmagnitude and for controlling the direction of incrementation of saidreference position towards or away from the final position.
 5. Thepositioning system of claim 4 wherein said actuator means iselectromagnetic and has a load acceleration/deceleration responserelated to an electric current supplied thereto for accelerating anddecelerating the movable member and said control system furthercomprises:feedforward means responsive to said scaling function forgenerating a feedforward control value having a magnitude correspondingto a magnitude profile for an actuator electric current modeled to movethe movable member to said final position using a modelelectromechanical actuator; andsaid means responsive to said positionerror value is further responsive to said feedforward control signal andcomprises: means for combining said position error value and saidfeedforward control value to generate a composite actuator signal tocontrol said actuator means.
 6. The positioning system of claim 5wherein said feedback means comprises:means for generating an errorsignal having at least one measurable characteristic varied in responseto the magnitude of the position error value; and wherein theincrementing means varies the incrementation rate in response to thevaried characteristic of the error signal.
 7. The positioning means ofclaim 6 wherein said feedback means further comprises:means formeasuring a variable of the movable member movement to indicate actualperformance of the positioning system; means for comparing the measuredvariable with a predetermined variable for a comparable movement by amodel positioning system; and means for varying said measurablecharacteristic of the error signal in response to the comparison.
 8. Thepositioning system of claim 7 further characterized by said controlsystem holding said movable member at said final position after movementby maintaining said reference position equal to said final position. 9.The positioning system of claim 1 wherein said means for dividingcomprises:a record medium along the defined path of movement; and servordata recorded on the record medium in a manner to permit said servo datato be sensed independently of any other detectable information on therecord medium, said servo data defining said plurality of segments andfurther indicating incremental position with respect to asegment;wherein said positioning system further comprises: servotransducer means coupled to said movable member for detecting the servodata recorded on the record medium;wherein said control system furthercomprises: means responsive to the servo transducer means for generatinga pair of position signals having values together indicatingunambiguously said actual incremental position; andwherein saidcontroller means is continuously responsive to the two position signalsand said position error value is at all times dependent upon the valuesof both of the two position signals.
 10. The positioning system of claim9 wherein said position error value is indicated by a position errorsignal and said controller means comprises:first memory means forgenerating a reference position signal having a value related to saidapproximate incremental position; and controller means for combining atleast one of said position signals with said reference position signalto form a first component of said position error signal.
 11. Thepositioning system of claim 10 wherein said pair of position signals arephase related to said actual incremental position and said referenceposition signal is phase related to said approximate incrementalposition and said controller means for combining comprises:first meansfor multiplying said reference position signal and said one positionsignal.
 12. The positioning system of claim 11 wherein said controllermeans further comprises:second memory means for generating a secondreference position signal phase related to said approximate incrementalposition; second means for multiplying said second reference positionsignal and the remaining position signal to form a second component ofsaid position error signal; and means for combining said first componentand said second component.
 13. The positioning system of claim 3 whereinsaid feedback means comprises:sign change means for controlling thepolarity of said position error signal in relation to relative proximityof the approximate incremental position and of the actual incrementalposition to the final position; and voltage controlled oscillator meansfor generating an error signal having a frequency varying about anominal frequency level in response to the polarity controlled positionerror signal from said sign change means.
 14. The positioning system ofclaim 13 wherein said updating means comprises:rate multiplier means forgenerating a cyclic signal having a cycling rate related to themagnitude of said scaling function and the frequency of said errorsignal.
 15. The positioning system of claim 14 wherein said means forgenerating a scaling function comprises:means for generating a distancesignal having a value indicating distance between stored referenceposition and said final position; and memory means for generating saidscaling function in response to said distance signal.
 16. In apositioning system for moving a movable member along a defined path ofmovement and including means for dividing the path of movement into aseries of contiguous segments and actuator means coupled to the movablemember for positioning of the member, a method of controlling themovement of the movable member from an initial position to a finalposition comprising the steps:generating an indication of actualincremental position of the movable member with respect to a segment;storing a reference position of the movable member, the stored referenceposition being variable, indicating an approximate position of themovable member with respect to the segments including an approximateincremental position with respect to a segment and initially indicatingsaid initial position; generating a position error value indicating adifference between said actual incremental position and said approximateincremental position; incrementing in response to a command indicatingsaid final position the stored reference position from said initialposition toward said final position; and actuating said actuator meansin response to said position error value.
 17. The method of claim 16wherein said incrementing step is repeated until said reference positionis incremented to said final position.
 18. The method of claim 17further comprising the steps of:generating a scaling function having amagnitude diminishing as said reference position is incremented to saidfinal position; and controlling the repetition rate of said incrementingstep in response to the magnitude of said scaling function.
 19. Themethod of claim 18 wherein said controlling step includes controllingsaid repetition rate in response to said position error value.
 20. Themethod of claim 19 further comprising the steps of:comparing themagnitude of the position error value with a predetermined value; andcontrolling the direction of incrementation of the reference position inresponse to the comparing step.
 21. The method of claim 20 furthercomprising the steps of:generating in response to said scaling functiona feedforward control value for having a magnitude corresponding to amagnitude for an actuator control signal modeled to move the movablemember to said final position using a model actuator means; andcombining said feedforward control value and said position error valueto generate a composite actuator control value; andwherein saidactuating step includes actuating said actuator in response to saidcomposite control value.
 22. The method of claim 21 wherein saidcontrolling step further comprises the steps of:measuring a variable ofthe movement of the movable member; comparing said measured variablewith a predicted variable value for a comparable movement of the memberby a model positioning system; and varying the repetition rate of saidincrementing step in further responsive said comparing step.
 23. Themethod of claim 22 wherein said controlling step includes the stepsof:generating an error signal having at least one measurablecharacteristic varying about a nominal value in response to themagnitude of said position error value; varying the characteristic ofthe error signal in response to the comparing step; and incrementing thereference position in response to the varied error signal.
 24. Themethod of claim 19 wherein said controlling step comprises the stepof:converting said position error value into a velocity error signal;and varying the repetition rate in response to said velocity errorsignal.
 25. The method of claim 24 wherein said controlling step furthercomprises the step of:measuring the movement of the movable member for apredetermined period of time; comparing the measured movement with apredetermined movement value representing a similar movement by a modelpositioning over said predetermined period of time; and varying thevelocity error signal in response to said comparing step beforecontrolling the repetition rate.
 26. The method of claim 25 wherein saidvelocity error signal has a varying frequency indicating velocity errorand said step of varying the repetition rate comprises:scaling thefrequency of the velocity error signal in proportion to the magnitude ofthe scaling function; and incrementing the reference position with thescaled velocity error signal.
 27. The method of claim 16 wherein saidmeans for dividing includes a record medium along the path of movementand servo information recorded on the record medium in a manner topermit the servor information to be sensed independently of any otherinformation recorded on the record medium, the recorded servoinformation defining said series of segments and indicating incrementalposition within a segment; wherein said system further includestransducer means coupled to the movable member for detecting said servoinformation; wherein said step of generating an indication of actualincremental position comprises the step of:generating a pair of positionsignals having values together indicating unambiguously an actualincremental position of the movable member with respect to a segment;andwherein the step of generating a position error value furtherincludes generating a position error value dependent at all times uponthe values of both of the two position signals.
 28. The method of claim27 wherein said step of generating a position error value comprises thesteps of:generating a first reference position signal in response tosaid reference position; and combining said first reference positionsignal and one of the two position signals.
 29. The method of claim 28wherein said pair of position signals are phase related to said actualincremental position, said reference position signal is phase related tosaid approximate incremental reference position and said step ofcombining comprises:multiplying the first reference position signal andthe one position signal to form a component signal.
 30. The method ofclaim 29 further comprising the steps of:generating a second referenceposition signal phase related to said approximate incremental position;multiplying said second reference position signal and the remainingposition signal to form a second component signal; and combining the twocomponent signals.
 31. A data storage system comprising:a data diskmounted for rotation; a transducer for detecting data recorded on saiddisk during rotation; a carriage mounting said transducer for movementradially with respect to the disk; a motor coupled to said carriage formoving said carriage radially with respect to said disk; servo datarecorded on the disk and defining a plurality of contiguous servo bandswhen the disk is rotated and indicative of position within the servobands, the transducer being capable of detecting the servo data recordedthereon; position detection means coupled to the output of thetransducer for generating a first transducer position signal and asecond transducer position signal having a fixed phase displacement withrespect to the first transducer position signal, the two transducerposition signals being phase related to an actual incremental positionof the servo transducer with respect to an opposing servo band;reference position means for storing a digital reference positionrepresenting an approximate position of the servo transducer withrespect to the servo bands including an approximate incremental positionwith respect to one band; controller means responsive to said twotransducer position signals and to the stored digital reference positionfor generating a position error signal indicating a difference betweenthe incremental position indicated by the two transducer positionsignals and an incremental position indicated by the digital referenceposition signal, the magnitude of the position error signal beingdependant at all time upon the magnitude of both of said two transducerpostion signals; and means responsive to said position error signal forsupplying a current to the motor for positioning said carriage.
 32. Thedata storage system of claim 31 wherein said controller meanscomprises:first digital memory means responsive to said referenceposition means for converting said stored reference position into afirst reference position signal phase related to said approximateincremental position; and multiplier means for multiplying said firstreference position signal by one of said transducer position signals togenerate a first component signal of the position error signal.
 33. Thedata storage system of claim 32 wherein said controller means furthercomprises:second memory means responsive to said reference positionmeans for generating a second reference position signal phase related tosaid approximate incremental position and having a fixed phasedisplacement with respect to the first reference position signal; secondmultiplier means for multiplying the second reference position signaland the remaining transducer position signal to generate a secondcomponent signal; and summing means for adding said first componentsignal and said second component signal to generate said position errorsignal.
 34. The data storage system of claim 31 furthercomprising:updating means responsive to a carriage position command forincrementing the stored reference position to a commanded position. 35.The data storage system of claim 31 wherein the updating meanscomprises:means responsive to said stored reference position and acommand signal indicating a commanded position for generating a velocityprofile function having a magnitude substantially proportional to amagnitude of a modeled velocity of the carriage when located at adistance from a movement final position equal to the distance betweenthe reference position and commanded position; and means forincrementing the stored digital reference position at rate related tothe magnitude of the velocity profile function.
 36. The data storagesystem of claim 35 wherein said updating means further comprises:firstmeans for controlling the direction of incrementation of the storedreference position towards or away from said commanded position.
 37. Thedata storage system of claim 36 wherein said position error signal isoscillatory in form and has a voltage magnitude of zero for zeroposition error and said means for controlling comprises:first means forcomparing the absolute magnitude of said position error signal apredetermined magnitude and means for incrementing the stored referenceposition away from the commanded position when said absolute positionerror magnitude exceeds said predetermined magnitude.
 38. The datastorage system of claim 37 wherein said updating means furthercomprises:means for controlling the polarity of the position errorsignal to indicate phase lead or lag between the reference position andthe actual incremental position with respect to the commanded position;a voltage controlled oscillator generating a cyclic signal having acycling frequency varying about a nominal frequency in response to thepolarity controlled position error signal; andwherein said means forincrementing, increments the stored reference position at a rate relatedto both the frequency of the cyclic signal and the magnitude of thevelocity profile function.
 39. The data storage system of claim 38wherein said updating means further comprises:means for measuringdistance moved by the carriage during a predetermined time interval ofan acceleration portion of a carriage movement; means for comparing themeasured distance with a predetermined distance value for a modelpositioning system accelerating a movable member for the samepredetermined time interval; and rate multiplying means between saidvoltage controlled oscillator and said means for incrementing forfurther varying the frequency of said cyclic signal in response to saidmeans for comparing.
 40. The data storage system of claim 38 whereinsaid updating means further comprises:second means for storing aninitial reference position value stored by the reference position meansat the beginning of a carriage movement; digital memory means responsiveto the second means for storing and the reference position means forgenerating a ratio value of a difference between a current referenceposition indicated by the reference position and the initial referenceposition value and a predetermined value for a maximum possible distancemoved in a predetermined time interval by a model motor-carriagecombination having a maximum modeled acceleration; times means forsignaling said predetermined time interval; storage means responsive tosaid timing means and said digital memory means for storing the ratiovalue existing at the end of said predetermined time interval; and ratemultiplying means between said voltage controlled oscillator and saidmeans for incrementing for reducing the frequency of said cyclic signalin proportion to said ratio.
 41. The data storage system of claim 35wherein said means is responsive to said stored reference position and acommand signal comprises:distance means responsive to said storedreference position for generating a distance signal having a valueindicating distance between the stored reference position and acommanded position and digital memory means responsive to said distancesignal for generating said velocity profile function.
 42. The datastorage system of claim 35 further comprising:memory means forgenerating a feedforward scaling function in response to said velocityprofile signal; switching means responsive to said components of theposition error signal for generating a reference voltage signal changingin polarity when said components of the position error signal becomesufficiently small in magnitude; and means for generating a feedforwardcontrol signal having a magnitude related to a magnitude of saidfeedforward control function and the polarity of said polarity signal.43. The data storage system of claim 42 wherein said means for supplyinga current to the motor further comprises:means for reducing gain of theposition error signal during movement of the carriage to the commandedposition; means for combining the gain controlled position error signaland the feedforward control signal to generate a composite controlsignal; and current means for supplying a current to the motor having amagnitude proportional to the magnitude of the composite control signal.44. In a data storage system incluuding a data disk mounted forrotation, a transducer for reading data recorded on said disks, acarriage mounting said transducer for movement in a radial directionwith respect to the disk, a motor coupled to the carrige for moving thecarriage radially with respect to the disk, the disk having recordedthereon servo signals for providing when the disk is rotated a pluralityof contiguous, concentric servo bands and indicative of position withinthe servo bands, the transducer being positioned opposite the data diskfor detecting the servo signals recorded thereon, a method ofcontrolling the positioning of the carriage and transducer with respectto the disks comprising the steps of:detecting the recorded servosignals; generating a first oscillatory position signal and a secondoscillatory position signal having the same form as the firstoscillatory position signal and a fixed phase difference from the firstoscillatory phase signal in response to said detecting step, the twooscillatory position signals indicating unambiguously actual incrementalposition of the servo transducer with respect to an opposing servo band;storing a variable reference position of the transducer; generating areference position signal having a magnitude related to the storedreference position and indicating an approximate position of the servotransducer with respect to the servo bands; generating a position errorsignal in response to said reference position signal, the firstoscillatory position signal and the second oscillatory position signal,the position error signal having a magnitude dependent at all times uponthe magnitudes of both the first oscillatory position signal and thesecond oscillatory position signal; and actuating said motor in responseto said position error signal.
 45. The method of claim 44 furthercomprising the step of:incrementing the stored reference position signalto cause movement of the carriage and transducer from an initialposition with respect to the servo bands to a commanded position withrespect to the servo bands.
 46. The method of claim 45 wherein saidvarying step comprises the steps of:generating a velocity profilefunction having a magnitude related to a magnitude of a modeled velocityof the carriage when located at a distance from a movement finalposition equal to the distance between the reference position and thecommanded position; and incrementing the stored reference position at arate related to the magnitude of the velocity profile function.
 47. Themethod of claim 46 wherein said method further comprises the stepof:generating a cyclic signal having a cycling frequency varying about anominal frequency in response to the magnitude of the position errorsignal; andsaid incrementing step further includes incrementing thestored reference position at a rate also related to the frequency of thecyclic signal.
 48. The method of claim 47 wherein said method furthercomprises the steps of:comparing the absolute magnitude of the positionerror signal to a predetermined value; and controlling the direction ofincrementation of the stored reference position in response to saidcomparing step.
 49. The method of claim 48 wherein said step ofgenerating a position error signal comprises the steps of:generatingfrom said reference position signal a pair of secondary referenceposition signals having oscillatory forms like the forms of the twooscillatory position signals and a fixed phase difference for indicatingunambiguously an approximate incremental position with respect to aband; and combining the two oscillatory position signals and the twosecondary reference position signals to generate said position errorsignal.
 50. The method of claim 49 wherein said method further comprisesthe steps of:measuring change in the reference position signal valueoccurring over a predetermined period of time; comparing the change to apredetermined value for a model positioning system including a modelmotor; and varying the frequency of the cyclic signal in response to thecomparing step before said incrementing step.
 51. The method of claim 50wherein the motor has a load-acceleration/deceleration response relatedto the magnitude of an electric current supplied thereto and said methodfurther comprises the steps of:generating a feedforward scaling functionin response to said velocity profile function; generating a feedforwardcontrol signal having a magnitude related to the magnitude of saidfeedforward scaling function; andsaid actuating step further comprisesthe steps of: reducing gain of said position error signal duringcarriage movement to the commanded position; combining said gaincontrolled position error signal and said feedforward control signal togenerate a composite control signal; and actuating said motor inresponse to said composite control signal.
 52. The method of claim 46wherein the actuator means is electromagnetic and has aload-acceleration/deceleration response related to the magnitude of anelectric current supplied thereto and said method further comprises thesteps of:generating a feedforward scaling function in response to saidvelocity profile function; generating a feedforward control signalhaving a magnitude related to the magnitude of said feedforward scalingfunction; andsaid actuating step further comprises the steps of:controlling gain of said position error signal; combining said gaincontrolled position error signal and said feedforward control signal togenerate a composite control signal; and actuating said motor inresponse to said composite control signal.
 53. In a servo system forcontrolling a movement of a movable member between an initial positionand a final position, the servo system generating a signal for feedbackcontrol during the movement having a signal variable indicative of anerror in the movement occurring during the movement, a motor adaptivecircuit comprising:means for measuring a variable of the movementoccurring during the movement; means for generating a predeterminedvalue of said variable for a comparable movement by a model servo systemand for comparing the measured variable with said predeterminedvariable; and means for varying said signal variable in response to saidmeans for comparing.
 54. The apparatus of claim 53 wherein said variableof the movement is position of the movable member, said value of thesignal is frequency and said variable of the movement is measured onlyan acceleration of the movable member.
 55. The apparatus of claim 54wherein the model servo system is modeled for the fastest possiblemovement between said initial position and final position, said meansfor comparing generates a ratio of a measured change of position to apredetermined change of position and said means for varying comprisesrate multiplier means for reducing the frequency of said signal inproportion to said ratio.
 56. The apparatus of claim 55 wherein saidmeans for measuring and means for generating together comprise;a latchstoring an initial position of the member; andmemory means responsive tosaid latch and to a signal indicating current position of the memberduring movement for generating ration having a value proportional to thedifference between current position and initial position.
 57. Theapparatus of claim 56 further comprising:means for timing anacceleration period of the movable member; latch means for storing saidratio when the acceleration period extends for a predetermined period oftime; and said rate multiplier means is responsive to the stored ratio.58. A method of adapting a servo control system using frequency varyingsignal to indicate error in a movement of a movable member comprisingthe steps of:measuring actual movement of the member for a predeterminedperiod of time; comparing the measured movement with a predeterminedmovement value representing the greatest possible movement achievableduring the predetermined period by a model servo system; and varying thefrequency of the error signal in response to said comparing step.